It is known that, under certain conditions, a "Karman vortex street" is shed in the wake of a bluff body when a flow of fluid is perpendicular to the body. The shedding of vortexes occurs periodically first from one side of the body and then from the other in accordance with the velocity of the flow. Thus, by counting the number of the vortexes created by the body in a unit time, the flow rate of the fluid can be measured.
Hitherto, a number of fluid flow rate measuring apparatuses have been presented by putting the above theory into practical use.